This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Nuclear Pore Complex (NPC, about 50 MDa) is the sole passageway for the transport of macromolecules across the nuclear envelope. The pore plays a key role in numerous critical cellular processes such as transcription, and many of its components are implicated in human diseases such as cancer. The recent work by the Sali group provided the first description of the macromolecular architecture of the yeast NPC. This structure defined the relative positions and proximities of its 456 constituent nucleoporin (nup) proteins, based on spatial restraints derived from experimental data. Further elucidation of the evolutionary origin and transport mechanism of the NPC will require higher resolution information. To help improve upon the resolution and accuracy of the NPC structure, we have begun small angle x-ray scattering studies at SSRL. Individial nup proteins and complexes are being expressed, produced and purified by our collaborators at Rockefeller University and Eli Lilly on a regular basis. We selected several nup proteins whose crystal structures have been solved to establish a benchmark. Among them is Nup145 which belongs to a highly conserved group of homologs found throughout the eukaryotes. Nup145N, the autoproteolised N-terminal half of the protein has been implicated in carrying RNA between the nucleus and the NPC by its analogy with Nup98. The crystal structures of Nup145N(443-605) and Nup98N, however, shows different modes of association within their respective crystallographic asymmetric units. Our preliminary solution x-ray scattering results demonstrate that Nup145N(443-605) forms the head-to-tail dimer in solution as observed in the asymmetric units of two different space groups.